1. Field of the Invention
This invention relates to the fabrication of devices and more particularly to the fabrication of junction devices.
2. Art Background
The physical intersection of two or more materials to form a junction is employed in many devices. For example, Josephson junctions are fabricated by depositing a superconducting material, oxidizing a surface of this material to produce an insulating layer, and depositing a second superconducting material over this insulating layer. The area of overlap of the two superconducting materials and the insulator forms a junction that has desirable electrical properties. It is often desirable to limit the area of the junction, i.e., the region of overlap. For example, in a Josephson device it is desirable to increase the conductance of the barrier to reduce hysteresis. This reduction of hysteresis is desirable since it generally yields faster switching times and simplifies the design of associated circuitry. However, to allow for greatly reduced hysteresis while maintaining the total device resistance at acceptable levels the junction area must be reduced significantly, i.e., reduced to smaller than 3.times.10.sup.-9 cm.sup.2 for typical devices.
Because submicron dimensions are required to produce the advantages associated with small area junctions, conventional lithographic techniques--techniques yielding dimensions on the order of 1 .mu.m--are not useful. Various expedients have been employed to reduce the dimensions obtainable with conventional lithography. For example, a method suitable for producing small area junctions, has been developed by R. H. Havemann. [See Journal of Vacuum Science and Technology, 15, 389 (1978).] This method involves the deposition on a substrate of a first layer, e.g., a layer of nickel, having a thickness corresponding to one dimension of the junction to be formed. An insulating material, SiO, is then deposited on the major surface of the first layer. (A resist is used to delineate the area on the substrate on which the first layer is to be deposited. Since the height of the resist is greater than that of the first layer, the resist also ensures that the insulating material is deposited only on the major surface and not on the edge of the first layer. Thus, the resist both delineates the first layer and also masks its edges.) The resist is then removed and the exposed edge of the first layer is oxidized to form a thin barrier layer. The oxide is then covered with a second layer of material such as nickel having a desired width delineated by a second resist mask. This width is smaller than the width of the first layer. In this manner a junction is formed in the region where the first layer, the oxide, and the second layer overlap. The area of this overlap region, and thus the area of the junction, is determined by the thickness of the first layer and the width of the second layer. Since layers as thin as 10 nm are producible by standard techniques, the fabrication of small area junctions is possible.
However, methods such as described above for reducing the junction area have certain shortcomings. For example, after the first layer and insulating layer are deposited, the substrate is taken from the deposition apparatus, the resist material is removed, and the second resist pattern is formed. The substrate is returned to the vacuum system for oxidation of the edge of the first layer. The second layer of material is then deposited through the second resist pattern. Exposure to the ambient environment allows contamination of the edge of the first layer where the junction is ultimately to be formed. Resist residues left after processing of the resist also contaminate this surface. The devices produced by forming junctions on this contaminated surface generally are not acceptable. In the case of Josephson devices, to maintain the junction resistance while reducing the junction area, the specific conductance of the oxide must be correspondingly increased. To achieve this result with contaminated surfaces extensive cleaning procedures are necessary to remove the contamination before forming the oxide. These cleaning procedures introduce additional processing complexities and often lead to further complications.